1. Field of the Invention
This invention relates to a process and apparatus for physically activating long-chain polymers, e.g. to change the conformation of starch molecules to increase their reactivity.
2. Description of the Prior Art
Starch is a high molecular-weight polymer of D-glucose units and is the principal reserve carbohydrate in plants. Most starches consist of a mixture of two types of polymers, namely, amylose and amylopectin associated by hydrogen bonding to varying degrees. Amylose is a linear polymer and may contain linear chains of several hundred glucose units linked together. In aqueous solution, the amylose chains are believed to be coiled in a helical structure. The amylopectin is a branched polymer of starch and may have a molecular size range from several hundred thousand to several millions.
Starch is, of course, one of the world's most abundant renewable raw materials, but suffers the problem of being difficult to react with most other compounds. It has long been known that major factors influencing its reactivity include the degree of intermolecular hydrogen bonding and the conformation of molecular units. More specifically, it has been known that factors which affect the reactivity of hydroxyl groups on the starch molecules are the electron density at the oxygen atom, steric interference and the presence of intra- and inter-molecular hydrogen bonds associated with the particular conformation assumed by the pyranose rings.
The literature on starch chemistry is filled with proposals for increasing the activity of starch. These among other things, can involve the introduction of additional free radicals on the starch molecules which provide reaction sites. Examples of activating techniques include both chemical and mechanical techniques, such as high energy irradiation, low energy irradiation in the presence of a synthesizing agent, redox systems and mechanical degradation.
It is also known particularly in the production of starch size solutions for use in the textile industry to make use of a mechanical shearing action in the treatment of starch. Thus, in a typical jet cooking apparatus used for producing starch sizes, starch slurry and steam under high pressure are supplied to a cooking valve. The starch slurry is metered into the cooking valve, in which the steam is allowed to impinge upon a thin film of starch. The heat gelatinizes the starch and the mechanical shearing action of the expanding steam disrupts the swollen granules. The cooked starch then is ready for the sizing operation.
Another example of shearing action on starch is to be found in Protzman et al, U.S. Pat. No. 3,137,592, dated June 16, 1964 where a gelatinized starch product is formed by passing a superficially dry mixture of starch and starch-swelling agent through a conventional screw-type plastics extruder at elevated temperature and pressure. This has been found effective in improving the rate of reaction between raw starch material and some starch-modifying agents. However, it is concerned primarily with the production of gelatinized starch products to be used in the traditional manner.
In trying to cross-link starch polymers to produce new polymeric materials and in graft co-polymerization, traditionally one has thought in terms of non-degradative reactions in which the basic characteristic of the starch is retained. It was found that starch breaks down quite readily with heating so that starch was traditionally thought of in terms of a quite heat-sensitive material which could not be subjected to severe reaction conditions.
It is an object of the present invention to provide a process for continuously converting a liquid slurry of starch, with or without modifying agents, into a homogeneous, modified or unmodified starch solution or suspension.
It is a further object to provide an apparatus suitable for carrying out the above process.